Imaging techniques that use the magnetic field response of an atom's nuclear spin or of its unpaired electron spin are known. These images depend on the responses generated by a sample placed within or contiguous to a probe both of which are located in a strong, static magnetic field. A number of electrical conductors comprise part of the probe and these conductors are typically arranged in a "birdcage" manner. A radio frequency signal is pulsed through the electrical conductors to generate a corresponding magnetic field that excites the nuclear spins of a group of protons that are aligned with the static field. As the group of excited protons return to the state in which they were prior to being excited by the magnetic field generated by the radio frequency, they produce a magnetic field response that induces electrical signals in the electrical conductors of the probe. This signal is picked up by image producing circuitry and is used to generate an image of the sample.
Through the application of specially modulated RF pulses and specifically modulated magnetic field gradients, the induced electrical signals produced by the magnetic field responses of the group of excited protons are indicative of the physical locations and chemical natures of the sample material under investigation. The presentation of the data as pixels (locations) of given brightness (signal intensities) provide an image of features within the sample. Similar imaging is also possible by using the responses of the unpaired magnetic moments (spins) of electrons of atoms to the radio frequency magnetic field produced by the radio frequency signal. This imaging is known as Electron Paramagnetic Resonance Imaging.
A commonly used probe type for Nuclear Magnetic Resonance imaging usually comprises one or more electrical conductors mounted about a sample cavity within a cylindrical body so the conductors enclose the cavity. The conductors are located in an ordered fashion in the vicinity of the sample cavity. Because a number of conductors are adjacent to each other, they possess an electrical mutual inductance that affects the electrical characteristics of the probe. For example, the mutual inductance of the conductors affects the impedance of the probe and the corresponding resonant frequency of the probe as a result. The resonant frequency of the probe is that frequency at which the capacitive and inductive attributes of the probe cancel one another so no phase shift occurs in an electrical signal transmitted through the probe. Capacitive tuning circuits may be used to tune the probe to a particular resonant frequency for the inherent inductance of the particular arrangement of conductors.
The typical nuclear magnetic resonance probe suffers from a number of limitations. For one, they are only effective at the resonant frequency or a few resonant frequencies for which the probe can be tuned. Second, the sample within or proximate to the sample cavity may also affect the electrical characteristics of the probe and alter the resonant frequency and/or the quality of the induced signals in the conductors of the probe. To return the probe to the selected resonant frequency, the probe must be retuned to the resonant frequency with the sample in the sample cavity. Another limitation of these imaging probes is the requirement that the sample fit within the sample cavity.
Other probes that are used in Nuclear Magnetic Resonance Imaging (MRI) are used at local surfaces. These include individual loops of wire or coils of wire placed adjacent to the material being imaged. These probes also require tuning to a particular resonant frequency through manipulation of their capacitative and inductive attributes to generate an effective image.
Another type of probe developed for NMR imaging is a catheter probe for imaging the walls of arteries and the like. Such a probe includes a dielectric material such as Teflon.RTM. about which electrical conductors are coiled. The electrical conductors are covered with a shrink sleeve to prevent contact of the vascular tissue in the artery with the electrical conductors. Because the electrical conductors are typically mounted about a cylindrical dielectric, they can be placed within an artery and maneuvered proximate a location of interest such as an arterial blockage. Once in place, a static magnetic field is produced to align the protons of the nuclei of the surrounding tissue and a radio or microwave frequency pulse is transmitted through the electrical conductors to produce the nuclear magnetic field response. In this manner, tissue surrounding a catheter probe can be imaged.
The catheter probe discussed above has a number of problems. For one, the inductance of the electrical conductors of the probe require a capacitive tuning circuit so the probe may be tuned to a resonant frequency. Moreover, the composition of the tissue surrounding the electrical conductors also affects the electrical characteristics of the probe and the tuning circuit may require adjustment for effective imaging. Finally, the field of view of the catheter probe is a function of the size of the coiled portion of the electrical conductors, and, for a given resonance frequency, the range of the capacitance needed to tune the probe can change with the length of the resonant portion.
What is needed is an imaging probe that can be used tier Nuclear Magnetic Resonance Imaging and Electron Paramagnetic Resonance Imaging which is not limited to a resonant frequency and which has an adjustable field of view and an adjustable depth of view.